Position detection is a known method in third generation mobile communication. In particular for code division multiple access (CDMA) mobile stations (MS), position detection using Advanced Forward Link Trilateration (AFLT) is known.
Advance Forward Link Trilateration is a geolocation technique utilizing the time difference of arrival (TDOA) or a time of arrival (TOA) of radio signals from the base stations measured by mobile stations.
To allow calculation of the position of mobile stations, in particular a code phase is detected. The code phase may be the fraction of the code period that has elapsed since the latest code boundary. By using the code phase, the time of arrival or the time difference of arrival can be calculated.
Cellular network-related wireless location methods can be subdivided into three categories according to the mobile station and network functionalities. These three categories are pure network-based methods, MS-assisted network-based methods, and MS-based network-assisted methods.
For a pure network-based method, the network fulfills all the positioning functionalities including location measuring and position calculations. An MS itself does not take any active part in the process. Obviously, these methods are applicable to legacy cellular phones. However, the network may require some modifications to accommodate a wide range of hardware products.
The second category, MS-assisted network-based methods, consists of methods, which require at least some active participation from the MSs. An MS can take part in location measuring or doing some other positioning-dedicated tasks, while most of the positioning functionalities are still completed in the network. The role of an MS is solely to assist the network in positioning.
In MS-based network-assisted methods, the roles of the MSs and the cellular network are reversed in comparison to those in the second category method. An MS makes location measurements and calculates its own position. Thus, the role of the network is simply to assist MSs in location estimation. Methods of this type enable a denser position-fixing rate.
The simplest method for locating a cellular phone is CELL-ID, which is based on cell identification. An MS can be assigned a location if the cell in which the MS is located can be identified. Since this is an inherent feature of all cellular systems, minimal changes to existing systems are needed. A cell only has to be associated with a location, such as by association with the coordinates of the base station of this cell.
This method boasts the additional advantage that no calculations are needed to obtain location information. Thus, the CELL-ID based method is fast and suitable for applications requiring high capacity. However, the drawback is that accuracy depends directly on cell radius, which can be very large, especially in rural areas.
To allow position detection, geological techniques like time difference of arrival and time of arrival are known.
Applying these techniques requires that the number of measurements is adequate to make an accurate estimation of the mobile station location. For instance, it is required that at least three TDOA measurements are available for uniquely determining the MS position. However, the underlying communication system is designed to reduce the interference by maintaining the transmission powers of the MSs and BSs at a minimum required level in order to accommodate more users. Consequently, only when the MS is at the edge of a cell, may it obtain enough measurements for location estimation. This is known as the hearability problem.
The angle of arrival (AOA)-based location method is one of the oldest positioning methods. Its early use began during the development of radar, sonar, and antenna array techniques. By means of array signal processing techniques, the direction of an MS with respect to BSs can be measured at BSs. Thus, the MS is at the intersection of the lines derived from AOA measurements. The accuracy of the AOA method is dependent on the distances between the MS to be located and the antenna arrays at BSs. The further the MS is from the antenna arrays, the larger is the positioning uncertainty. Non-line-of-sight (NLOS) signal propagation is a significant source of inaccuracy. When NLOS components exist, AOA measurements will be distorted, thus resulting in degraded positioning accuracy.
The measurements required in Time of Arrival (TOA) methods are the absolute signal transmission times between MS and BSs that are equivalent to MS-BS distances. The MS is located at the intersection of several circles, of which the centres are the BSs used, and the radii are the measured MS-BS distances. At least three TOA measurements are required to uniquely determine the 2-D position of an MS.
TOA wireless location methods require that all base stations (BS) be precisely synchronized to each other and that the MS to be located be synchronized to the network as well. For this reason, TOA positioning is feasible only in fully synchronized networks; for example, in IS-95 CDMA systems.
The measurements in Time Difference of Arrival (TDOA) methods are relative signal transmission times, which are equivalent to distance differences. A TDOA measurement defines a hyperbola with two BSs as the foci. At least three hyperbolae are needed for unique MS position determination.
A TDOA method requires that all base stations involved be synchronized. This can be done either by synchronizing all BSs physically, or by bringing all BSs to a common reference time by measuring time differences between BSs. MSs do not need to be synchronized since the MS clock bias is the same with respect to all BSs and differencing any two TOA measurements will cancel out the MS clock bias. It may also be possible to provide the BSs with a time reference from a satellite based positioning signal, such as GPS.
In a CDMA System, a pilot phase signal is continuously transmitted. This signal allows mobile stations to detect the presence of CDMA channels and provides timing information for demodulation. According to mobile communication standard, the pilot signal is a DS spread spectrum signal. The spread function is a zero Walsh function. The signal is further modulated by a pseudo noise (PN) sequence of a particular base station. The pseudo noise sequences of different base stations only differ in an offset, which is a multiple of 64 pseudo noise chips.
In particular, there are 64 physical channels in the forward link of an IS-95 CDMA cellular system; these are distinguished by the 64 orthogonal Walsh functions, which serve as digital carriers. These physical channels form four types of logical channels.
First, the pilot channel is identified by Walsh function zero. It continuously broadcasts a known signal to provide the MSs a robust time, frequency, and phase reference for demodulation in other channels.
The pilot channel possesses dominant transmission power. Approximately 15-20% of the maximum transmission power of a BS is dedicated to the pilot channel to ensure the visibility of the pilot signal over the coverage area. This also makes pilot signals more easily acquired from neighboring cells as well. The pilot signal is a known continuous broadcasting signal. It enables an MS to keep locked on the pilot Pseudo Noise (PN) code. All BSs transmit the same PN sequence but with different offsets. This makes it easier in the search process of a receiver to acquire TDOA measurements.
The process of generating a pilot signal provides a zero Walsh function with a chip rate of 1.2288 Mcps (mega chips per second). It is first modulated by the pilot baseband “data”. Then, this intermediate signal is separated into an I-component and a Q-component to further modulate the I-channel PN sequence and the Q-channel PN sequence. Wave shaping, amplification, and RF carrier modulation are finally conducted to generate the actual signal transmitted to MSs.
The Walsh code is one type of orthogonal code. It is used in IS-95 CDMA systems to separate different physical channels. Both the I-channel PN sequence and the Q-channel PN sequence are maximal length sequences generated by 15-stage shift registers and lengthened by the insertion of one chip per period in a specific location in the sequences. Thus, the sequence length is in chips. Each base station is distinguished by a different phase offset in both the I-channel and the Q-channel PN sequences. The offset is a multiple of 64 PN chips, which yields 512 possible 64-chip offsets. At a rate of 1.2288 Mcps, the I-sequence and Q-sequence repeat every 26.66 ms, or 75 times every 2 seconds.
The synchronization channel is identified by a Walsh function, and is a continuously broadcasting channel. It provides MSs with BS timing information, cell site identification number, and other information for synchronization.
In addition, there can be up to seven paging channels. A paging channel contains paging messages and conveys other control messages from the BSs to the MSs.
Eventually, there are at least 55 traffic channels. They carry user information. They also carry control messages using “blank and burst”, which is a time multiplexing technique used on traffic channels to send overhead signaling or (optionally) secondary traffic in which a frame of primary digital voice data is blanked, i.e., not transmitted to allow the overhead or secondary traffic to be transmitted at a 9600 bit/second rate. In the blank-and-burst format, a frame or frames of primary digital voice data are suppressed or not transmitted to make time available to send signaling traffic. The digital voice frames are lost. However, the degradation of the recovered analog voice is minimal provided not too many frames are blanked consecutively. Typically, the transmitter controllers wait for a less-than-full-rate Vocoder frame in which signaling traffic can be multiplexed with voice traffic without the loss of any voice bits.
Multiplexing signaling with voice in less-than-full-rate frames is called dim and burst, which is a time multiplexing technique used on traffic channels to send overhead signaling or (optionally) secondary traffic in which a less-than-full-rate frame of primary digital voice data and overhead or (optionally) secondary traffic data are combined and transmitted at a 9600 bit/second rate.
To allow location estimation, a method called forward link location is known. Reception on forward link location is performed coherently. To maintain coherence, the MS searches for and locks onto a pilot pseudo noise (PN) sequence. Every base station sector broadcasts the pilot PN sequence with a unique known offset, as previously described. The base stations are synchronized, allowing the MS to identify the signal originating from a particular BS sector.
A common IS-95 mobile terminal has four rake receiver fingers, three of which are used to receive an incoming signal and one to search for multipath signals and handover candidates. During operation, the terminal keeps track of the strongest pilot channel in its vicinity. When requested by the BS through a pilot measurement request order (PMRO), the BS will report all pilot signals it receives above a given threshold. The message sent back to the BS includes the magnitude of each pilot, relative to the offset of the base station transmitting the pilot signal.
The pilot signal may be used within the BS as phase reference. Knowing the PN offsets of the pilots transmitted from nearby BSs it is possible to construct TDOA estimates for the BS. As long as the MS is able to detect at least three pilot signals from three different BSs, the location estimation may be possible.
The main problems in location estimation using these methods are synchronization errors in the BSs. These errors result from poor PN resolution and multipath or non-line-of-sight propagation of the received signals.
In particular the specifications 3GGP2 C.S0022-0, v1.0, “Location Service (position determination service)”, and 3GPP2 C.S0036-0, v.0, “Recommended Minimum Performance Specifications for C.S0022-0 Spread Spectrum Mobile Stations” describe methods allowing code division multiple access (CDMA) systems to calculate positions of mobile equipment. Depending on where the position calculation is performed, the method may either be mobile station based or mobile station assisted. In a mobile station based solution, the position may be calculated within the mobile phone itself. In the mobile station assisted case, a position may be calculated in a network server, in particular in a position determination entity. The position determination entity may use for its position calculation information provided by the mobile station, in particular phase and time measurements reported by the mobile station.
To allow calculation of a position using a time of arrival or a time difference of arrival method, at least three different pilot phase signals and pilot phase measurements should be provided. The pilot phase measurements are carried out within the mobile stations.
For position calculation within the mobile station, assistance data from the network is required. This assistance data may include the co-ordinates of the reference and neighboring base stations together with their time corrections.
A drawback of known AFLT position measurements using pilot phase measurements is that the pilot phase signal is subject to noise. The noise level causes the pilot phase signal to be disturbed, thus deteriorating the measured pilot phase.